1. Field of the Invention
The present invention relates generally to the conversion of heat energy into electrical energy, and more particularly to a method and system for transporting hydrogen ions through a selective electrolyte under a chemical potential gradient to produce electricity.
2. Description of the Background Art
The conversion of chemical energy to electrical energy may be accomplished in a variety of ways. Most commonly, electrochemical cells and batteries rely on redox reactions involving the transfer of electrons from the substance being oxidized to the substance being reduced. By carrying out the reaction in such a way that the reactants do not come into direct contact with each other, it is possible to cause the electrons to flow through an external circuit where they can be used to perform work.
Although invaluable for a number of applications, electrochemical cells do suffer from certain drawbacks. In particular, such cells have a finite life resulting from the exhaustion of the reactants. Although most cells can be recharged by applying a reverse-polarity voltage across the electrodes, such recharging requires a separate electrical source and prevents the continuous operation of the cell over indefinite periods.
To overcome these problems, fuel cells were developed. In general, fuel cells operate by passing an ionizable species across a selective electrolyte which blocks passage of the non-ionized species. By placing porous electrodes on either side of the electrolyte, a current may be induced in an external circuit connecting the electrodes.
The most common fuel cell is the hydrogenoxygen fuel cell where hydrogen is passed through one of the electrodes while oxygen is passed through the other electrode. The hydrogen and oxygen combine at the electrolyte-electrode interface to produce water. By continuously removing the water, a concentration gradient is maintained to induce the flow of hydrogen and oxygen into the cell. Fuel cells of this type have been particularly valuable in manned space flights where they not only provide relatively large amounts of electricity, but also supply drinking water for the personnel.
Despite their usefulness, fuel cells of the type described above suffer from a number of disadvantages. First of all, the fuel cells require a continuous supply of reactant in order to continue to produce electricity. Related to this, the cells also produce a continuous product stream which must be removed. Although disposal of the water produced by hydrogenoxygen fuel cells is seldom a problem, the removal of the product of other fuel cell systems is not always as simple. The second problem relates to the selection and maintenance of the porous electrodes. Electrodes must be permeable to the reactant species entering the cell. Over time, however, such porous electrodes frequently become fouled and plugged so that migration of the reactants through the membrane is slowed. Such slowing results in the reduced production of electricity. Third, the selection of an appropriate electrolyte is not always easy. The electrolyte, which may be a solid electrolyte, must rapidly transport the ionized species in order to increase the current production. Frequently, the limited migration of the ionized species through the electrolyte is a limiting factor on the amount of current produced.
For these reasons, it would be desirable to provide fuel cells which do not require a continuous source of reactants in order to operate. In particular, it would be desirable if the fuel cells could operate with reactants which are regenerated by means of an alternate energy source, preferably heat. Such thermoelectric conversion cells will preferably utilize electrodes and electrolytes which do not become fouled or plugged and which provide for rapid migration of the ionizable species. Finally, such thermoelectric conversion cells will display high current to weight ratios allowing for their utilization in applications where volume and weight are critical, such as space flight.
Certain thermoelectric conversion cells have been proposed. See. e.g., U.S. Pat. No. 3,458,356, where molten sodium is induced to flow across a solid electrolyte by a pressure gradient induced by a temperature gradient. The electrolyte is chosen to selectively pass sodium ions, and a current is generated as sodium atoms lose electrons on entering the electrolyte and gain electrons on leaving the electrolyte. The cell is workable, but suffers from plugging of the porous electrodes required to pass sodium ions. Moreover, diffusion of the sodium ions through the solid electrolytes is relatively slow, limiting the amount of current available from the cell.
Thermally regenerative fuel cells are also described in U.S. Pat. Nos. 3,357,860 and 3,119,723. The following patents are also of interest: U.S. Pat. Nos. 3,014,048; 3,031,518; 3,192,070; 3,338,749; 3,368,921; 3,511,715; 3,817,791; 4,049,877; and 4,443,522.